ipv4: Allow configuring subnets as local addresses

This patch allows a host to be configured to respond to any address in
a specified range as if it were local, without actually needing to
configure the address on an interface.  This is done through routing
table configuration.  For instance, to configure a host to respond
to any address in 10.1/16 received on eth0 as a local address we can do:

ip rule add from all iif eth0 lookup 200
ip route add local 10.1/16 dev lo proto kernel scope host src 127.0.0.1 table 200

This host is now reachable by any 10.1/16 address (route lookup on
input for packets received on eth0 can find the route).  On output, the
rule will not be matched so that this host can still send packets to
10.1/16 (not sent on loopback).  Presumably, external routing can be
configured to make sense out of this.

To make this work, we needed to modify the logic in finding the
interface which is assigned a given source address for output
(dev_ip_find).  We perform a normal fib_lookup instead of just a
lookup on the local table, and in the lookup we ignore the input
interface for matching.

This patch is useful to implement IP-anycast for subnets of virtual
addresses.
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/flow.h

@@ -49,6 +49,7 @@ struct flowi {
 	__u8	proto;
 	__u8	flags;
 #define FLOWI_FLAG_ANYSRC 0x01
+#define FLOWI_FLAG_MATCH_ANY_IIF 0x02
 	union {
 		struct {
 			__be16	sport;

net/core/fib_rules.c

@@ -182,7 +182,8 @@ static int fib_rule_match(struct fib_rule *rule, struct fib_rules_ops *ops,
 {
 	int ret = 0;
 
-	if (rule->iifindex && (rule->iifindex != fl->iif))
+	if (rule->iifindex && (rule->iifindex != fl->iif) &&
+	    !(fl->flags & FLOWI_FLAG_MATCH_ANY_IIF))
 		goto out;
 
 	if (rule->oifindex && (rule->oifindex != fl->oif))

net/ipv4/fib_frontend.c

@@ -153,17 +153,16 @@ static void fib_flush(struct net *net)
 
 struct net_device * ip_dev_find(struct net *net, __be32 addr)
 {
-	struct flowi fl = { .nl_u = { .ip4_u = { .daddr = addr } } };
+	struct flowi fl = { .nl_u = { .ip4_u = { .daddr = addr } },
+			    .flags = FLOWI_FLAG_MATCH_ANY_IIF };
 	struct fib_result res;
 	struct net_device *dev = NULL;
-	struct fib_table *local_table;
 
 #ifdef CONFIG_IP_MULTIPLE_TABLES
 	res.r = NULL;
 #endif
 
-	local_table = fib_get_table(net, RT_TABLE_LOCAL);
-	if (!local_table || fib_table_lookup(local_table, &fl, &res))
+	if (fib_lookup(net, &fl, &res))
 		return NULL;
 	if (res.type != RTN_LOCAL)
 		goto out;